DESCRIPTION: (Applicant's Abstract) This project seeks a clearer understanding of both the postsynaptic factors that shape fast IPSCs and EPSCs as well as the mechanism by which synaptic inputs are integrated on the dendrites of hippocampal neurons. One set of experiments will determine the impact of GABAA and AMPA receptor desensitization on synaptic transmission. The physiological impact of desensitization has been inferred, in large part, by extrapolation from the results of kinetic studies of receptors in excised outside-out (O-O) membrane patches. The implicit assumption is that the kinetic properties of synaptic receptors can be faithfully modeled by those of receptors in O-O patches. However, this assumption has yet to be directly tested for AMPA and GABAA receptors. Indeed, our preliminary data suggest that the process of patch isolation alters the desensitization properties of both AMPA and GABAA receptors, with the effect on the latter receptors being especially marked. A second set of experiments will determine the mechanism by which synaptic inputs separated in time and space are combined by the dendrite, a fundamental issue in neurobiology that remains poorly understood. Our preliminary data suggest that summation on dendrites cannot be adequately described by a single process similar to that demonstrated in classic studies of the muscle endplate. Instead, we postulate that summation readily switches between different modes (such as linear, sublinear, and supralinear). We propose that such mode switching is a defining feature of temporal and spatial summation by dendrites of neurons in the central nervous system. Hippocampal neurons in dissociated cultures and in organotypic slice cultures will be studied. Postsynaptic responses will be recorded with patch clamp techniques. Glutamate and GABA will be applied to dendritic receptors in situ with the use of a novel method of rapid focal photolysis that we have recently developed. This innovation has enabled us to overcome the major technical problems that had rendered these fundamental issues previously inaccessible.